Electrical Transport and Quantum Phase Transitions of Ultrathin Films of Metals Prepared by in Situ Low-Temperature Deposition.

Electrical transport properties of films of metals have been studied in films sufficiently thin that they could be considered to be two dimensional. These films were prepared by successive deposition in situ of vaporized metals supplied by effusion cells onto substrates held at liquid helium temperatures. A number of investigations were carried out. In the first study, films of Bi, Pb and Al grown on amorphous Ge substrates exhibited a crossover from insulating to superconducting behavior as the film thickness gradually increased. The temperature dependent electrical conductivities of Bi films were found to scale accurately with a single parameter which vanished when approaching the onset of superconductivity. These observations suggest a direct phase transition from an insulator to a superconductor at zero temperature. The critical exponents and resistance, both of which should be universal quantities, were inferred. The latter were found to be essentially the same as the normal sheet resistance threshold for the onset of superconductivity, which was found to be very close to the quantum resistance for pairs, h/4e ^2 = 6455Omega. Films of Pb were also found to scale although with a noticeable scattering of data points. For Al films, the scaling was even less accurate. In a second study, nonsuperconducting amorphous Pd films were also fabricated. Their electrical conductivities were also found to scale accurately with a single parameter. However, the scaling parameter, which can be related to a localization length, vanishes with the controlling parameter exponentially rather than algebraically, suggesting a marginal insulator-metal transition at zero temperature in two dimensions. In a third study, superconducting fluctuation effects were studied in Ge/Ag/Ge sandwich structures. This work suggested the possibility of superconductivity at the Ag/Ge interface. The final investigation was concerned with the study of an unusual temperature dependence of the resistive tails of disordered superconducting Pb, Al, and Bi films in small magnetic fields. It was argued that the observed results were due to quantum tunnelling of vortices. These studies have provided physical insight into a number of important features of two-dimensional disordered electronic systems.